Geochemical and geochronological constraints on the Mesoproterozoic Red Granite Suite, Kunene AMCG Complex of Angola and Namibia

Abstract

Anorthosite-mangerite-charnockite-granite (AMCG) suites form large batholiths emplaced during the Proterozoic Eon. Magma source(s), emplacement mechanisms, and the tectonic setting of AMCG suites remain poorly understood. We present new major and trace element geochemistry combined with U-Pb zircon geochronology and Lu-Hf isotopes for the Red Granite Suite, part of the Mesoproterozoic Kunene AMCG Complex (Angola and Namibia), and its Paleoproterozoic host rocks, to elucidate the petrogenesis and tectonic setting of the Kunene Complex granitoids. The studied samples are mostly granite with minor syenite and quartz monzonite. LA-MC-ICPMS U-Pb zircon dates indicate that felsic magmatism was active for at least 90 Myr (1450–1360 Ma) and shows spatial and temporal similarities with emplacement of the main anorthositic body. The geochemistry of the granitoids did not change substantially over time, with predominant alkali-calcic, A-type granites. The εHf(t) of zircon grains in the Red Granite Suite ranges between −11.3 and + 1.6. These results, when considered together with Hf isotopic compositions of zircon from the Paleoproterozoic host rocks (εHf(t) = -11.3 to + 0.4) and xenocrysts in the Red Granite, suggest the suite is derived from the mixing of a Paleoproterozoic crustal component with juvenile mantle-derived material. Syn-magmatic east–west shortening has been documented in both the anorthosites and Red Granites at the southwestern margin of the Kunene Complex in the period 1400–1380 Ma. The results from the Hf isotopes on zircon, combined with the long-lived nature of magmatism, the syn-contractional magmatism, at least locally, and the requirement for extensive crustal melting, suggest that the Red Granite Suite was formed in a convergent margin setting. This challenges the traditional view that A-type granitoids are restricted to anorogenic, extensional settings, and is in agreement with recent views on the petrogenesis of massif-type anorthosites and their associated MCG magmatism.